R&D tax incentives (RDTIs) – here defined as R&D tax credits and other special incentives – must be viewed as the fiscal translation of conflicting goals among all players in the public and private sectors. A comparative analysis of the various national mechanisms among OECD countries as well as a focus on the French experiences will help us to identify and to understand the different types of constraints that RDTIs have to satisfy. We thus intend to show that it does exist national fiscal systems of innovation that are almost neglected in standard NSI frameworks despite their growing weight in national S&T policies.